1. Field of the Invention
The present invention relates to an acceleration sensor which uses resistance elements and to a method for producing such an acceleration sensor. More particularly, the present invention relates to an acceleration sensor for detecting mechanical deformation in terms of variations in electrical resistance introduced by resistance elements formed in a working mechanism. In such a configuration, deformation is caused by an acceleration applied to the working mechanism which, in a preferred embodiment, is formed of a semiconductor substrate material.
2. Discussion of the Related Art
Sensors are known for detecting acceleration, force and the like by obtaining variations in electrical resistance that resistance elements undergo when working mechanisms in the sensors are subjected to elastic deformation due to the action of acceleration, force or the like exerted on the working mechanisms. In such sensors, a thin-walled portion capable of elastic deformation is formed as the working mechanism on a semiconductor substrate in the form of a plate of single crystal silicon. The resistance elements are also formed in the working mechanism.
FIG. 8 is a plan view of a conventional acceleration sensor having a circular diaphragm portion. FIG. 9 is a sectional view of the sensor shown in FIG. 8.
In FIGS. 8 and 9, an acceleration sensor 50 has a working mechanism 52 in the form of a plate of single crystal silicon as well as a pedestal 51. The working mechanism 52 has a thin-walled diaphragm 52D in its center. A weight portion 53 is coupled to the center of the diaphragm 52D and, upon undergoing acceleration, is displaced vertically and bilaterally in relation to the view shown in FIG. 9. With the displacement of the weight portion 53, the thin-walled diaphragm 52D is also displaced vertically and bilaterally. Resistance elements provided for the diaphragm 52D detect the displacement in terms of variations in electrical resistance, whereby acceleration is detected.
The diaphragm portion is often designed so that its outer periphery is made circular. This is intended not only to attain highly sensitive properties, but also to secure reliability and mechanical strength of the sensor of the sort described above.
Although the thin-walled diaphragm portion is formed by etching from a plate of single crystal silicon, various techniques have been devised to implement circular etching because the etching rate differs according to the crystalline orientation of the plate of single crystal silicon.
For example, the thickness of the diaphragm portion has been regulated by first applying isotropic etching and then skillfully combining anisotropic etching and an etch stop technique. Therefore, the method of forming the diaphragm portion tends to become complicated and the problem is that such a method makes it difficult to improve the yield and tends to increase processing time. In other words, the conventional method requires increased levels of skill on the part of the worker.
Consequently, one technique has been introduced in which the diaphragm portion is formed by anisotropic etching only. FIG. 10 is a plan view of a working mechanism 60 of an acceleration sensor having a square diaphragm portion 62 and a pedestal 61 which is positioned in the lower part thereof.
Another technique that has been proposed is to subject the (110) crystal face of a plate of single crystal silicon to anisotropic etching using an octagonal mask pattern.
FIG. 11 is a diagram illustrating stress distribution in the acceleration sensor of FIG. 10 based on the finite element method. Analysis under the finite element method (FEM analysis) is one of the simulation techniques used for analyzing structural properties, such as stress distribution, in each part in accordance with the solution of convergence obtained through numerical analysis, such as successive calculation and the like, by dividing an object to be examined into extremely small elements, creating mathematical models element by element, and assigning each model a space-temporal boundary condition together with an initial condition.
FIG. 11 shows the results of calculation of the distribution of stresses applied to the square diaphragm portion 62 of FIG. 10 formed with a wafer of single crystal silicon.
As shown in FIG. 11, when acceleration is exerted in the direction of G, stresses concentrate on parts perpendicular to the direction G out of the outer periphery of the diaphragm portion 62 and appear as stresses st1, st2. Other stresses concentrate on parts perpendicular thereto out of the periphery of the central square of the diaphragm portion 62 and appear as stresses st3, st4.
FIG. 12 is a plan view illustrative of the structure of the working mechanism 70 of an acceleration sensor prepared by employing the aforementioned technique. As shown in FIG. 12, an octagonal diaphragm portion 72 is formed on the working mechanism 70 in the form of a plate of single crystal silicon having a rectangular plane.
In an attempt to implement the aforementioned techniques of producing acceleration sensors and, more specifically, where such a working mechanism has the square diaphragm portion 62 of FIG. 10, for example, it is essential to make the area of the square diaphragm portion 62 equal to or greater than that of the aforementioned circular diaphragm portion (52D of FIG. 8) in order to secure sensitivity equal to that which is available from the circular diaphragm portion. Therefore, the area of the square diaphragm portion 62 is maximized to the extent possible within the working mechanism of limited dimensions. However, this can result in connection failures as the pedestal-to-diaphragm coupling area decreases.
On the other hand, an anisotropic etching interface S71 in the working mechanism 70 having the octagonal diaphragm portion 72 of FIG. 12 forms an angle .theta.2 as small as approximately 35.degree. as shown in FIG. 13; this is also the case with the interface S73. Therefore, the dimensions of the working mechanism 70 have to be reduced to secure the pedestal-to-diaphragm coupling area. Actually, the top of a working mechanism measuring 8 mm by 6.8 mm respectively representing the horizontal length L2 and the vertical length L3 as shown in FIG. 12 is not reducible. This is disadvantageous and still poses a problem in that the L2 and L3 constitute an obstacle to reducing the size of the acceleration sensor. Moreover, the number of acceleration sensors to be cut out of a wafer tends to decrease and the disadvantage again is that such acceleration sensors become costly.